There is a need for a curative that effectively cures a novolac resin without emitting ammonia during the process. There is a further need for a curative that does not require an extreme elevated temperature to cure a novolac resin. In all cases the curative should provide a state of cure such that the cured novolac resin has useful physical and chemical properties.
Novolacs are thermoplastics that find application in a wide variety of manufactured products. Novolacs bind foundry aggregate or refractory materials. They find use in any number of finished goods, such as brake linings, acoustic insulation, bonded felt, molding compounds, or structural composites.
Generally, because it is thermoplastic, the novolac is cured. It can be the major component of a finished product, as is the case with the general class of thermoplastics. However, the novolac is widely used as the binding matrix of composite materials.
It is well known in the art that the extent of cure the novolac undergoes determines, in part, the quality of the product made with the novolac. The extent of cure determines the thermal resistance, chemical resistance, and structural strength of products made using novolac resins. Generally, these properties will improve as the extent of cure increases. The extent cure increases as the number of reactions between curative and novolac resin increases. Inadequate cure of the novolac will compromise the temperature resistance, chemical resistance, and structural strength of the finished product.
Articles manufactured using a novolac binder typically must offer resistance to the effects of elevated temperatures. If used as a refractory binder, for example, the novolac cannot flow or degrade at the temperatures under which refractory shapes find application. If used as a foundry binder, the novolac must at least resist the temperatures of molten metals long enough for the cast-metal product to take its shape. Similar requirements can exist for other applications where novolacs bind composites such as brake linings or even structural composites. Accordingly, the curative selected must be capable of providing a product that has cured properties such that the product meets these rigorous demands.
Materials manufactured using a novolac binder typically must offer chemical resistance. In refractory applications, for example, the refractory article may be coated with an auxiliary coating designed to increase the refractoriness of the combination. The novolac resin binding the article cannot deteriorate on contact with the coating. The value of the refractory article is lost if the novolac resin deteriorates. Similarly, it is typical that foundry cores and molds have an auxiliary coating applied to improve the refractoriness of the core or mold. It is important that the binder holding the core or mold together resist chemical attack by the coating. Chemical resistance is a requirement of other novolac-bound manufactured articles.
It should be apparent to those skilled in the art that articles manufactured from novolacs or articles made by binding materials with novolacs must also possess structural integrity. Accordingly, the curative selected must be one that allows the manufacture of a product that has the necessary level of structural strength.
As a binder, a novolac coats the other materials that form the finished article. This will sometimes require that the novolac be a liquid at the temperatures of application. Accordingly, at some temperature, the novolac resin must have a viscosity that facilitates the coating of the other materials forming the finished article. Once coated, the article may be shaped and then cured, typically, by using a curative and possibly heat.
Because novolacs typically cure at temperatures above room temperature, the emission of volatile compounds during the curing step can be a concern. The elevated temperatures will increase the emission of volatile compounds.
Several novolac curatives are known in the art. Typically, formaldehyde, hexamethylenetetramine, or a melamine resin convert the novolac to an insoluble infusible condition. Hexamethylenetetramine, or Hexa, or HMTA, is a well known novolac curative. In Phenolic Resins, Chemistry, Applications and Performance, (A. Knop and L. A. Pilato, Springer-Verlag (1985)), the authors describe novolac curing as requiring "a crosslinking compound which is mainly HMTA, and rarely paraformaldehyde or trioxane." Where HMTA or formaldehyde cure the novolac resin, emission of volatile reaction products will occur during the cure reaction. When the curative is HMTA, ammonia evolves during curing of the novolac resin. Furthermore, curatives like HMTA typically require curing temperatures as high as 150.degree. C.
While the use of melamine resin as a novolac curative does not result in the release of ammonia during the cure reaction, its use is not without disadvantages. For example, melamine resins typically require either an acid catalyst or elevated temperatures to cure a novolac resin. Also, melamine resin curatives tend to be slower than HMTA and produce a lesser extent of cure.
Liquid alkoxylated melamines or methylolated melamine resins generally require the application of heat, acid catalyst, or both, to effect a reasonable rate of cure. But, melamine curatives do find use and are sold commercially, for example, under the CYMEL trademark (products of Cytec Industries, Inc.), the RESIMINE trademark (products of Monsanto Chemical Co.), and the CASCOMEL trademark (products of Borden Chemical, Inc.). In fact, cross-linking reactions with alkoxylated melamine products may not occur at all without the use of an acid catalyst. See, for example, Cytec Industries product bulletin entitled "Melamine Crosslinking Agents, Performance Property Trends Based on Functional Groups," that describes the general need for an acid catalyst in order to achieve practical cure speeds with melamine curatives. However, some methylolated melamine curatives do not require the application of acid catalysts to effect cure of a novolac, but would require heat.
It is generally known that acids or thermally produced latent acids will lower the curing temperature of novolacs. See, for example, Phenolic Resins, Chemistry, Applications and Performance, (A. Knop and L. A. Pilato, Springer-Verlag (1985)), where it is reported that the rate of reaction between HMTA and a novolac increases with decreasing pH. These same acids will also strongly catalyze the cure of melamine resins.
U.S. Pat. No. 5,648,404 to Gerber discloses the use of lower alkoxylated melamine-formaldehyde resin curing agents (triazine hardeners). The disclosure of this patent is incorporated by reference in its entirety. The triazine hardeners of Gerber have a high temperature of activation, and preferably 80% of the cure occurs at 200.degree. C. and above. These curatives find use in application to hot refractory surfaces. The curatives of Gerber are particularly useful in refractory tap-hole applications for blast furnaces. Because of their high temperature of application, these curatives understandably have high temperatures of activation.
Onium salts have been used as curatives for novolac resins. In U.S. Pat. No. 5,254,664, Narang claims onium salts as catalysts to the crosslinking of poly(2-oxazoline) compounds with, for example, aromatic hydroxy compounds such as novolac resins.
Benzoxazine may be an intermediate product in the reaction of HMTA and phenol or substituted phenols. See, for example, Phenolic Resins, Chemistry, Applications and Performance, (A. Knop and L. A. Pilato, Springer-Verlag (1985)). However, there is no suggestion that benzoxazine is useful as a curative or even participates in the further reaction of HMTA and phenol or substituted phenols. In fact, is suggested that the crosslinking reaction involves hydrolysis of HMTA, by trace amounts of water in the novolac, thus forming .alpha.-aminoalcohols. The .alpha.-aminoalcohols are then converted to carbonium ions due to the presence of the acidic phenate. The alcoholates are then free to react with phenol or substituted phenols, via Mannich reaction, to form benzylamine compounds.
Until now, benzoxazines were considered to be poor novolac curatives. There are no known commercial uses of these benzoxazines as novolac curatives. But benzoxazines, as evidenced by the prior art, tended to be synthesized from phenolic compounds that were not polymeric in nature and were never resoles. With few exceptions, generally the prior art discloses the use of phenolic compounds that are small molecules relative to resoles. Higginbottom, in U.S. Pat. No. 4,501,864, describes the synthesis of benzoxazines using polyphenols that are generally simple diols. Higginbottom suggests the use of novolac resins in the synthesis of benzoxazines. However, Higginbottom does not disclose the use of resoles in benzoxazine synthesis. Similarly, Thrane, in U.S. Pat. No. 4,719,253, discloses the use of bisphenol to synthesize benzoxazine. In U.S. Pat. No. 5,543,516 to Ishida, the inventor discloses mono- and di-functional phenols, but polyvinyl phenol is the only reference to a polymeric form of phenol useable in the synthesis of benzoxazine.
There are several until now unanswered needs relating to novolac curatives. A need exists for a curative that will not emit ammonia during the cure reaction while at the same time it neither requires the acid catalysts and/or elevated temperatures of melamine resins nor the extreme elevated temperatures of lower-alkoxylated triazines. A still further need exists for a curative that provides a cure sufficient to provide adequate thermal, chemical, and structural properties, while possessing the advantages of no ammonia emission, broad pH range of application, and conventional temperatures for cure activation.
The benzoxazine polymer composition disclosed herein is also a thermosetting resin. This polymer will undergo homocondensation at elevated temperatures. It is therefore also useful solely as a thermosetting resin. Such applications include use as a laminating or coating resin, as a binder for refractory materials and foundry aggregate, and as a binder for felt and fiber. Because of its general chemical structure, it is anticipated that the benzoxazine polymer composition disclosed herein will have improved high temperature resistance and chemical resistance as compared to conventional phenolic thermosets.
Benzoxazines have been used as polymerics. Higginbottom, in U.S. Pat. No. 4,501,864, discloses a polymerizable composition comprising a poly(3,4-dihydro-3-substituted-1,3-benzoxazine) and a reactive polyamine or polyamine generating compound, useful as a potting, encapsulating, and laminating resin, and as a surface coating. The dihydrobenzoxazine compound reacts with a polyamine compound to form a cured polymer. Similarly, U.S. Pat. No. 4,719,253, to Thrane, discloses a self-curable composition comprising benzoxazine and a secondary amine. In both references, benzoxazine reacts with an amine to form the cured product and as such are not thermosetting compounds. Benzoxazines also have been used in the preparation of carbon-carbon composites. In U.S. Pat. No. 5,152,939 and U.S. Pat. No. 5,266,695, both to Ishida, the pyrolysis product of multifunctional benzoxazine compounds form the carbon-carbon structure.
Until now, benzoxazines were prepared in solutions of organic solvents or in solventless systems. In U.S. Pat. No. 4,501,864, Higginbottom discloses a process of synthesizing benzoxazine in an organic solvent solution even when aqueous formaldehyde is a reactant. Similarly, Thrane, in U.S. Pat. No. 4,719,253, discloses the use of non-reactive organic solvents even when it is desirable to produce a water dispersible benzoxazine. In U.S. Pat. No. 5,543,516 to Ishida, the inventor uses no solvent, save for the solvency reactants may have for each other, in a method for preparing benzoxazines. Ishida found that water is a distinct disadvantage to many applications employing benzoxazine. Ishida describes the prior art wherein benzoxazine is synthesized in a suitable organic solvent such as dioxan, toluene or alcohol.